Valve actuation in an internal combustion engine is required in order for the engine to produce positive power, as well as to produce engine braking. During positive power, intake valves may be opened to admit fuel and air into a cylinder for combustion. The exhaust valves may be opened to allow combustion gas to escape from the cylinder.
During engine braking, the exhaust valves may be selectively opened to convert, at least temporarily, an internal combustion engine of compression-ignition type into an air compressor. In doing so, the engine develops retarding horsepower to help slow the vehicle down. This can provide the operator increased control over the vehicle and substantially reduce wear on the service brakes of the vehicle. A properly designed and adjusted compression release-type engine brake can develop retarding horsepower that is a substantial portion of the operating horsepower developed by the engine in positive power.
The braking power of a compression release-type engine brake may be increased by selectively opening the exhaust valves to carry out exhaust gas recirculation (EGR) in combination with compression release braking. Exhaust gas recirculation denotes the process of briefly opening the exhaust valve near bottom dead center on the intake stroke of the piston. Opening of the exhaust valve at this time permits higher pressure exhaust gas from the exhaust manifold to recirculate back into the cylinder. The recirculation of exhaust gas increases the total gas mass in the cylinder at time of the subsequent compression release event, thereby increasing the braking effect realized by the compression release event.
For both positive power and engine braking applications, the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams makes it difficult to adjust the timings and/or amounts of engine valve lift needed to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a "lost motion" device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage means. In a lost motion system, a cam lobe may provide the "maximum" (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded fully, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. An example of such a system and method is provided in co-pending U.S. application Ser. No. 08/701,451 filed Aug. 22, 1996, Ser. No. 08/512,528 filed Aug. 8, 1995 (now abandoned), and in Hu U.S. Pat. No. 5,537,976, which are assigned to the same assignee as the present application, and which are incorporated herein by reference.
In the lost motion system of Applicant's co-pending application, an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may be a solenoid valve and a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston.
Many lost motion systems have not had the combined capability of providing an adequate fail-safe or "limp home" mode of operation and of providing variable degrees of valve lift over an entire range of cam lobe positions. In previous lost motion systems, a leaky hydraulic circuit could disable the master piston's ability to open its associated valve(s). If a large enough number of valves cannot be opened at all, the engine cannot be operated. Therefore, it is important to provide a lost motion system which enables the engine to operate at some minimum level (i.e. at a limp home level) should the hydraulic circuit of such a system develop a leak. A limp home mode of operation may be provided by using a lost motion system which still transmits a portion of the cam motion to the valve after the hydraulic circuit therefor leaks or the control thereof is lost. In this manner the most extreme portions of a cam profile can still be used to get some valve actuation after control over the variable length of the lost motion system is lost and the system has contracted to a minimum length. The foregoing assumes, of course, that the lost motion system is constructed such that it will assume a fully contracted position should control over it be lost and that the valve train will provide the minimum valve actuation necessary to operate the engine when the system is fully contracted. In this manner the lost motion system may be designed to allow the engine to operate, albeit not optimally, so that an operator can still "limp home" and make repairs.
Kruger, U.S. Pat. No. 5,451,029 (Sep. 19, 1995), for a Variable Valve Control Arrangement, assigned to Volkswagen AG, discloses a lost motion system which when fully contracted may provide some valve actuation. Kruger does not, however, disclose that the lost motion system may be designed such as to provide limp home capability. Kruger rather discloses a lost motion system which starts from a fully contracted position upon every cycle of the engine. The lost motion system thereby provides a base level of valve actuation when fully contracted, such base level being modifiable only after the lost motion system has been displaced a predetermined distance. It follows therefore that the Kruger lost motion system is undesirably limited to starting from a fully contracted position each engine cycle and cannot vary the amount of lost motion until after the lost motion system has been displaced by a cam motion.
Many lost motion systems have also typically not utilized high speed mechanisms to rapidly vary the length of the lost motion system. Lost motion systems have accordingly not been variable such that they may assume more than one length during a single cam lobe motion, or even during one cycle of the engine. By using a high speed mechanism to vary the length of the lost motion system, more precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions.
Applicant has determined that the lost motion system and method of the present invention may be particularly useful in engines requiring valve actuation for positive power, compression release engine braking, and exhaust gas recirculation valve events. Typically, compression release and exhaust gas recirculation events involve much less valve lift than do positive power related valve events. Compression release and exhaust gas recirculation events may, however, require very high pressures and temperatures to occur in the engine. Accordingly, if left uncontrolled (which may occur with the failure of a lost motion system), compression release and exhaust gas recirculation could result in pressure or temperature damage to an engine at higher operating speeds. Therefore, Applicant has determined that it may be beneficial to have a lost motion system which is capable of providing control over positive power, compression release, and exhaust gas recirculation events, and which will provide only positive power or some low level of compression release and exhaust gas recirculation valve events, should the lost motion system fail.
An example of a lost motion system and method used to obtain retarding and exhaust gas recirculation is provided by the Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) for a Method And A Device For Engine Braking A Four Stroke Internal Combustion Engine, assigned to AB Volvo, and incorporated herein by reference. Gobert discloses a method of conducting exhaust gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the inlet stroke. Gobert uses a lost motion system to enable and disable retarding and exhaust gas recirculation, but such system is not variable within an engine cycle.
Previous lost motion systems or method, routinely have not enabled precise control of valve actuation to optimize valve movement for different engine operating conditions, while maintaining an acceptable limp home capability. Furthermore, the lost motion systems or methods of that are known do not teach or suggest the use of a high speed lost motion system capable of varying the amount of lost motion during a valve event such that the system independently controls valve opening and closing times, while maintaining an acceptable limp home capability. Such independent control may be realized by modifying a standard cam lobe initiated valve opening event with precise amounts of lost motion, which may range between a minimum and maximum amount at different times during the valve event. In addition, none of the prior art discloses, teaches or suggests any system or method for defaulting to a predetermined level of positive power valve actuation (which may or may not include some exhaust gas recirculation) should control of a lost motion system be lost.
Accordingly, there is a significant need for a system and method of controlling lost motion which: (i) optimizes engine operation under various engine operating conditions; (ii) provides precise control of lost motion; (iii) provides acceptable limp home capability; and (iv) provides for high speed variation of the length of a lost motion system.
Known systems for providing lost motion valve actuation have also tended to be non-integrated devices which add considerable bulk to the valve train. As vehicle dimension have decreased, so have engine compartment sizes. Accordingly, there is a need for a less bulky lost motion system, and in particular for a system which is compact and has a relatively low profile.
Furthermore, there is a need for low profile lost motion systems capable of varying valve actuation responsive engine and ambient conditions. Variable actuation of intake and exhaust valves in an internal combustion engine may be useful for all of the aforementioned valve events (positive power and engine braking). When the engine is in positive power mode, variation of the opening and closing times of intake and exhaust valves may be used to modify valve opening and closing times in an attempt to optimize fuel efficiency, power, exhaust cleanliness, exhaust noise, etc., for particular engine and ambient conditions. During engine braking, variable valve actuation may enhance braking power and decrease engine stress and noise by modifying valve actuation as a function of engine and ambient conditions.